In the end I found the solution. I've added these lines on top of the file : /home/ziomario/Scrivania/PassT-Cubic/Debian-new/custom-disk/boot/grub/grub.cfg,with this content :
menuentry "Debian GNU/Linux Custom (kernel 5.10.0-18-amd64)" {
linux /d-i/gtk/vmlinuz APPEND file=/cdrom/preseed/preseed.cfg auto=true initrd=/live/initrd.gz boot=live components locales=en_US.UTF-8 quiet splash "${loopback}"
initrd /d-i/gtk/initrd.gz
}
and the preseed.cfg file has been taken into consideration. Imagination is more powerful than knowledge.
Mario.